When a signal is fed to two loudspeakers equally distant from a listener, and symmetrically placed before him, the resulting sound seems to come from a location midway between the loudspeakers. If the listener moves closer to one of the loudspeakers, the apparent location of the source of sound also moves in the same direction. If the difference in distance is sufficiently large, the sound will appear to come only from the nearer loudspeaker. This effect limits the effectiveness of two channel sterophonic sound reproduction for listeners not approximately equidistant from the two loudspeakers.
A practical result of the effect has been the inability of the cinema industry to use two channel stereophonic sound systems in conjunction with film exhibition. In this application, it is highly desirable that the voices of the actors should appear in the great majority of scenes to come from the center of the screen, which is the location as their images, for members of the audience seated in all parts of the theatre. Because of the effect described above, listeners near the screen and away from the center line of the auditorium will hear the actors' voices as coming from the side, rather than the center of the screen. As a result, cinema exhibition, while universally employing an optical sound track for single-channel sound, has made use of three or more tracks for stereophonic sound reproduction. Multiple-track recordings to provide three or more tracks are normally made by the application of a magnetic recording medium to the film, a process which greatly increases the cost of the prints used for exhibition. Furthermore, since magnetic and optical sound tracks cannot be played on the same apparatus, exhibitors must add equipment to their threatres to play the magnetically recorded tracks, and distributors of films must maintain stocks of both types to service the requirements of different exhibitors. This is particularly unfortunate because two-channel sterophonic sound tracks can be made optically, and the resulting films could be satisfactorily projected as single-channel programs in all theatres, were it not essential to provide a stable central image, only obtainable from a third channel.
It has been proposed to sum the two signals from the channels of a two-channel sound track to provide a center-channel signal which is fed to a center-channel amplifer and loudspeaker. In particular, U.S. Pat. No. 2,819,348, issued to B.P. Bogert on Jan. 7, 1958, describes a system in which a delay is introduced into the center channel so that the left and right loudspeakers are enabled by the known procedence effect to establish a stereo image. It is stated that this enables the left and right loudspeakers to handle little power, not necessarily of good quality, yet still establish the stereo image, while a high quality center channel amplifier and loudspeaker establish the main impression of sound quality and volume.
It can be seen, however, that such an arrangement will establish the stero image incorrectly for listeners appreciably nearer to one of the left and right loudspeakers than the other.